Cellular telephony is based on the concept of dividing a geographical area into a plurality of individual sub-areas or "cells." Situated within each cell is a base station containing at least one receiver and transmitter pair (i.e., a radio channel unit) tuned to a particular frequency (channel) for communicating with a mobile terminal, also tuned to receive and transmit on that channel. Typically, the mobile terminal is carried in a vehicle in transit within that cell. As the vehicle containing the mobile terminal travels from one cell into another, the call between the mobile terminal and the cell site base station is "handed-off" (i.e., transferred) to the base station of the new cell.
In order to determine whether the mobile terminal remains in communication with the base station, or if not, whether to hand-off the call from one base station to another, the cell site transmits a Supervisory Audio Tone (SAT) signal superimposed on its voice/data signal and the mobile terminal transponds the SAT signal back to the cell site. For so long as the base station receives the SAT signal, the base station knows that it is in communication with the mobile terminal. As described in my previous U.S. Pat. No. 5,001,742, issued Mar. 9, 1991, and assigned to AT&T Bell Laboratories, the instant assignee, the SAT signal presence is currently detected by monitoring the power of the signal. When the power of the SAT signal exceeds a predetermined threshold, the SAT signal is deemed to be present. Conversely, if the power of the SAT signal is below the threshold, the SAT signal is deemed to be absent.
While this method is effective for detecting the presence of the signal, in practice, the time required to detect the SAT signal in this manner has been found to be excessive. In order to reliably detect the presence of an SAT signal by monitoring its power, the signal power measurement must be reliable and stable. As a consequence, some processing delay is invariably involved. The delay incurred in detecting the presence of an SAT signal tends to cause long mute times during call hand-offs, which is disadvantageous. When a call is to be handed off between base stations, the mobile-to-land portion of the call is muted briefly to eliminate undesirable, unquiet receiver noise when the mobile terminal switches channels.
At the base station, the radio channel unit commences the muting when the mobile terminal confirms the hand-off order on the old channel. The radio channel unit at the new base station unmutes the voice signal when it confirms the presence of the SAT signal from the mobile terminal on the new channel. The length of the muting interval is determined by the time required to detect the presence of the SAT signal.
The length of time spent in detecting the absence of an SAT signal, such as when a mobile terminal abnormally terminates a call, is also disadvantageous, as it may lead to excessive noise. When a mobile terminal abnormally terminates a call, the land-based party will hear unquiet receiver noise before the base station senses the absence of the SAT signal and terminates the call. The longer the time required to detect the absence of the SAT signal, the longer the duration of the noise.
Thus, there is a need for a technique for more rapidly detecting the presence and absence of an SAT signal.